The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling the start of changing of the transmission ratio for upshifting.
A known control system for a continuously variable belt-drive transmission comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependence on driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to the servo devices, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to decide the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The line pressure is controlled to prevent the belt from slipping on pulleys in order to transmit the output of the engine.
At the start of the vehicle, the transmission ratio is set at a maximum value. When the vehicle speed and engine speed reach set values under a driving condition, the transmission ratio starts to change (to upshift). The transmission ratio is automatically and continuously reduced.
In the control system, the transmission ratio control valve is operated to change the transmission ratio in dependence on a balance between force of a spring dependent on the magnitude of the depression of the accelerator pedal and a pivot pressure dependent on the engine speed. Accordingly, the transmission is upshifted so that the vehicle speed increases although the engine speed is constant. However, the upshift starts when the engine speed reaches a considerably high speed. Such an upshift pattern is undesirable in driveability. Therefore, it is preferable to modulate the operation of the transmission ratio control valve to lower the upshift starting speed and to upshift the transmission at a low engine speed in a low vehicle speed range.
In order to meet such a requirement, Japanese Patent Laid Open Nos. 59-159456 (U.S. Pat. No. 4,565,110), and 60-159455 disclose a transmission ratio control valve having a modulator plunger in a spool thereof. A regulator spring is provided between the modulator plunger which is positioned dependent on the line pressure and an operating plunger which is shifted in accordance with the magnitude of the depression of the accelerator pedal. Thus, at the start of the changing of the transmission ratio, since a high line pressure is applied to the modulator plunger, the force of the regulator spring is reduced so that the spool is balanced at a low engine speed. Accordingly, the upshifting starts at a low engine speed. As the transmission is upshifted, the line pressure is reduced, thereby projecting the plunger to increase the force of the regulator spring. Thus, the engine speed at which the spool is balanced becomes progressively higher. In FIG. 6, the dotted line shows the modulated change of the upshifting at a wide-open throttle as a modulator range .DELTA.N.
The modulator range .DELTA.N is decided dependent on the difference between a maximum line pressure and a minimum line pressure in the transmission system. On the other hand, the line pressure depends on the engine torque, that is on the displacement of the engine. Therefore, in a motor vehicle engine having a small displacement, the line pressure is inevitably low so that the modulator range .DELTA.N becomes small.
If the modulator range .DELTA.N is small, the upshift starting engine speed becomes high.
On the other hand, when the kickdown is operated at a point A in FIG. 6, the engine speed is raised to a point B'. The difference in engine speed .DELTA.N' is so large that a shock occurs. In addition, although the engine speed increases, vehicle speed does not increase.